1. Field of the Invention
The invention relates to charge injection devices (CID) for optical sensing and more particularly to an improved readout circuit for an optical sensing CID array facilitating an extended dynamic range.
2. Prior Art
Optical sensing CID arrays are well known. They may take the form of area arrays or linear arrays and may respond to visible light, or to light of longer wavelength than visible wavelengths such as infrared (IR). In linear IR sensing arrays, the sensor substrate material is often Indium Antimonide (InSb) or mercury cadmium telluride (HgCdTe). These materials are compound semiconductors which are doped to achieve a desired impurity level. When exposed to IR, photon collisions create electron-hole pairs in the substrate. In the usual construction, a common electrode is applied to the under surface of the substrate, and an oxide layer is applied to the upper surface of the substrate, followed by a plurality of individual transparent electrodes each associated with a particular sensor element. The sensor element, with its insulated electrode, when suitably reversely biased, stores IR induced charges (the holes) in a "potential well", to use the conventional description.
Conventionally, all sensor elements are continuously exposed to the optical flux, so that when a "potential well" is present in the element, optically induced charges are stored. If the charges accumulate with the bias disconnected, the accumulating charge reduces the depth of the potential well. This reduction in depth may be directly sensed over a given period of time while the bias is disconnected and used as an indication of the intensity of the optical flux. However, this is not ordinarily done, because there is a low limit to the time that charge may be accumulated with reasonable linearity, and therefore this mode of operationa has a low limit to usable sensitivity.
A convenient charge integration time for optimum sensitivity in the time required to readout all the elements of the array in succession. Ideally, the optically induced charges should be allowed to accumulate on each element for the total period required to read out all the other elements (i.e. the line time approximately) before returning to the same sensor element for a second readout.
The period required for a line is the time required for the readout of all n sensor elements, i.e. the multiple of the time for taking a reading at a single sensor element (i.e. the pixel time) multiplied by the number of such elements or pixels. The pixel time is set by the time constants of the sensor element and the readout circuitry. In each pixel time: time is allocated for resetting the sensor element and the readout circuitry to a standard value (VR); time is allocated for taking a first sample prior to injection of the accumulated charge into the sensor substrate; time is allocated for charge injection, and finally time is allocated for taking a second sample, before going on to the next element.
The known readout technique which is described in the copending U.S. application Ser. No. 811,474 (35-EL-1676) of Messrs Wang, Swab, Winn and Gibbons, filed Dec. 20, 1985, and entitled "A READ AND CLEAR READOUT CIRCUIT AND METHOD OF OPERATION OF AN IR SENSING CHARGE INJECTION DEVICE", allows for charge integration over the line time, the charge integration process being linearized by "resetting" the sensor element to a suitable reverse bias at the pixel rate. The periodic resetting allows image charges to flow to the gate to reduce the reduction in depth of the potential well produced by the charge accumulated during the prior pixel level when the sensor element was "floating". When the known readout technique is optimized, a linear dynamic range of three orders of magnitude may be achieved.
Granting that the dynamic range of the readout circuitry is optimized for low level signals, the readout circuit is overloaded when strong signals are present. Overloading is a disadvantage which the present invention seeks to avoid.